The invention relates to a silk screen printing paste which is provided according to the thick-layer technique and is to be fired in a non-oxidizing atmosphere and which comprises a mixture of a vitreous passive component as a permanent binder, an active component and a temporary organic binder. The invention also relates to hybrid circuits manufactured by means of said paste.
The invention is used in the field of micro-electronics, more in particular in the manufacture of hybrid micro-circuits for which a whole series of silk screen printing pastes is to be formulated which are adapted to the various carriers used.
Such silk screen printing pastes are known, for example, from British Patent Specification 1,489,031 which discloses a conductive silk screen printing paste comprising copper which paste is provided on a carrier of sintered aluminum oxide and which is curable in a non-oxidizing atmosphere, i.e. an atmosphere having an oxygen content below 0.001%.
When such sintered aluminum oxide carrier is used, it is possible to formulate a whole series of suitable pastes which are to be fired in a non-oxidizing atmosphere at temperature of approximately 850.degree. C.
The recent marketing of certain new carriers, for example, the black enameled sheet steel of Messrs. ALPHAMETALS necessarily forces manufacturers to formulate new series of suitable pastes which are curable at lower temperatures, approximately 550.degree. C., so as not to spoil the enamel of the carrier.
Black enameled sheet steel consists of an extra-low carbon steel core, coated with an enamel layer consisting of filled glass: the glass containing essentially various transition metal oxides, such as Co, Mn, Ni . . . and the filler being, for example, SiO.sub.2, ZrO.sub.2, ZrSiO.sub.4 . . .
The enamel is applied as frit slurry, then dried and fired to form the coating.
Such a substrate offers several unique features of interest to hybrid manufacturers. Enameled steel is strong and easy to form, and may serve either as a carrier of circuitry or as a structural component or chassis. Special shapes and holes in the steel may be formed prior to enameling. The steel core is an excellent heat sink, and may serve as an electrical plane, usually as the ground, or sometimes as a power distribution plane.
Lead oxide-containing glasses have a low melting-point, but they suffer from the disadvantage that upon firing in a neutral atmosphere--for example in nitrogen--at a temperature above 450.degree. C. they decompose by reduction of the lead oxide by the active component.
Moreover, the organic binder of the silk screen printing paste fails to disappear fully from the silk screened layer, in particular when a neutral firing atmosphere is used, and causes the formation of residues which spoil the properties of such layers and contribute to the decomposition of the glass.
Further order to prevent the decomposition of the glass a composition is chosen which contains no reducing oxides at the firing temperatures in question. However, such a composition has a higher melting-temperature, in the order of 900.degree. C., which requires use of carriers of sintered aluminium oxide which can withstand said firing temperatures.
In order to better eliminate the organic carrier, Applicants have considered to introduce metal oxides of a valency of more than 1 in the silk screen printing paste the valency which metal oxides, during firing, are reduced to monovalent and which are dispersed in small quantities in the organic carrier, for example, the oxides of praseodymium (Pr.sub.6 O.sub.11, PrO.sub.2), cerium (CeO.sub.2) or cobalt (Co.sub.3 O.sub.4), when said oxides are reduced to a lower degree of valency they, change into the vitreous phase without adversely influencing the properties of the silk screened layer. This improvement is disclosed in British Patent Specification 2,037,270.
The glass used in the this British Patent Specification is within the following composition range in mol.%:
SiO.sub.2 : 5-20% PA1 B.sub.2 O.sub.3 : 15-40% PA1 Al.sub.2 O.sub.3 : 0-5% PA1 ZnO: 25-40% PA1 CaO+MgO: 5-30% PA1 Li.sub.2 O: 0-10%. PA1 PbO: 40-60% PA1 SiO.sub.2 : 25-45% PA1 Al.sub.2 O.sub.3 : 5-15% PA1 B.sub.2 O.sub.3 : 0-10%. PA1 PbO: 57% PA1 SiO.sub.2 : 33% PA1 Al.sub.2 O.sub.3 : 10%.
This glass comprises no lead oxide (PbO) and has a comparatively high annealing point of 575.degree. C.
U.S. Pat. No. 3,647,532 discloses a copper-containing conductive silk screen printing paste, the glass of which comprises lead oxide; this necessitates firing in a weakly oxidizing atmosphere with a partial oxygen pressure between 10.sup.-1 and 10.sup.-4 at. so as to void decomposition of the glass. Continuous and accurate control of the gas atmosphere introduced in the firing furnace of the silk screen printed patterns is required. After firing in such an atmosphere the glass moreover shows a poor bonding.